Prefabricated panels are commonly used to form ceilings or walls and in many other places. Such panels are commonly made of sheets of material, for example aluminum alloy, and it is generally desirable to reduce the thickness of the panelling material so as to reduce its cost and weight. Reduction of thickness is however restricted by the necessity to retain sufficient strength and rigidity. It is also desirable that the panel is cheap to manufacture and easy to assemble in numerous different applications.